1. Field of the Invention
One mode of this disclosed invention relates to semiconductor devices and a manufacturing method thereof. Further, one mode of the present invention relates to semiconductor devices which control a relatively large amount of electrical power.
2. Description of the Related Art
An energy gap of a silicon carbide semiconductor (4H—SiC) is 3.02 eV, which is about three times that of a silicon semiconductor, 1.12 eV. A dielectric breakdown electric field of a silicon semiconductor is 0.3 MV/cm while a dielectric breakdown electric field of a silicon carbide semiconductor is 3.5 MV/cm, which is more than ten times as high as that of a silicon semiconductor. Therefore, the use of a silicon carbide semiconductor for electrical devices typified by inverters and converters is expected.
In spite of those advantages, commercialization of transistors utilizing a silicon carbide semiconductor has not been realized because of low quality of silicon carbide semiconductor substrates and difficulty in increasing a size of the substrates.
Specifically, it has been found that defects of a silicon carbide substrate should be reduced. There is a problem in that a silicon carbide substrate has hollow-core defects called micropipes and only one micropipe in a transistor prevents the transistor from functioning. Besides the hollow-core defects, a silicon carbide substrate has many defects such as dislocation. In order to manufacture a high-quality silicon carbide substrate, a high-quality silicon carbide semiconductor layer is tried to be grown on a semiconductor substrate by homoepitaxial growth and heteroepitaxial growth, for example (see Patent Documents 1 and 2).
Although attempts have been made to improve quality of a silicon carbide substrate, a problem of a small size of the substrate is still to be solved. The size of a single crystal silicon substrate is 12 inch. On the other hand, the size of a silicon carbide substrate is still 3 inch generally. This is because crystal growth of a silicon carbide semiconductor is difficult to control. After all, although devices including a silicon carbide semiconductor are expected to be high performance devices, their commercialization is delayed in practice due to low quality of crystals and low productivity.    [Patent Document 1] Japanese Published Patent Application No. 2006-032655    [Patent Document 2] Japanese Published Patent Application No. 2006-036613